Scope modifiers: Difference between revisions
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(→{{header|Common Lisp}}: Add example using (declaim (special *bug*)).) |
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The defining operators <code>defvar</code> and <code>defparameter</code> globally declare a variable special, though this can also be done using <code>declaim</code>. Local special declarations are rarely used. |
The defining operators <code>defvar</code> and <code>defparameter</code> globally declare a variable special, though this can also be done using <code>declaim</code>. Local special declarations are rarely used. |
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The next example declaims that <code>*bug*</code> has dynamic scope. Meanwhile, <code>shape</code> has lexical scope. |
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[[Category:Common Lisp examples needing attention]] [This task should be expanded with some examples.] |
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<lang lisp>;; *bug* shall have a dynamic binding. |
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(declaim (special *bug*)) |
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(let ((shape "triangle") (*bug* "ant")) |
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(defun speak () |
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(format t "~% There is some ~A in my ~A!" *bug* shape)) |
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(format t "~%Put ~A in your ~A..." *bug* shape) |
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(speak)) |
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(let ((shape "circle") (*bug* "cockroach")) |
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(format t "~%Put ~A in your ~A..." *bug* shape) |
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(speak))</lang> |
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The global function <code>speak</code> tries to use both <code>*bug*</code> and <code>shape</code>. For lexical scope, the value comes from where the program ''defines'' <code>speak</code>. For dynamic scope, the value comes from where the program ''calls'' <code>speak</code>. So <code>speak</code> always uses the same "triangle", but can use a different bug. |
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<pre>Put ant in your triangle... |
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There is some ant in my triangle! |
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Put cockroach in your circle... |
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There is some cockroach in my triangle!</pre> |
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The stars around <code>*bug*</code> are only good style; they are no requirement of the language. Lispers like to put stars on special variables. |
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=={{header|E}}== |
=={{header|E}}== |
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